The present invention relates to a voltage converter with a variable input and output and including transformer means from the secondary winding of which a dc voltage is derived.
A converter of that type is already known in the art. Therein, the secondary winding of the transformer means is generally coupled to the output via a diode, whilst a capacitor is connected across this output. In this known converter, the voltage drop across the diode leads to significant reduction of the converter efficiency. This is particularly true for low output voltage application and is not acceptable for applications where the efficiency is a critical parameter as for instance a Solid State Power Amplifier (SSPA) which is an essential element of any telecommunication satellite payload that amplifies the RF signal and sends it to the antenna.
To overcome the poor efficiency of the known diode-rectifying converter, synchronous rectifying converters are used.
Therefore, the present invention more particularly relates to a synchronous rectifying converter with a variable input and output, said converter including transformer means having a main secondary winding and a control secondary winding, said main secondary winding having a first main terminal coupled to a first output terminal of said converter, and a second main terminal coupled via a controlled electronic switch to a second output terminal of said converter, and said control secondary winding having a first control terminal coupled to said main secondary winding, and a second control terminal coupled to a control input terminal of said switch, a capacitor being coupled between the first and the second output terminal of said converter.
Such a synchronous rectifying converter using a controlled electronic switch instead of a diode is also already known in the art. The advantage with respect to the diode-rectifying converter is that it presents a smaller voltage drop at the cost of an additional control secondary winding used to control the electronic switch. However, the field of applications of the synchronous rectifying converter is more restricted than that of the diode-rectifying converter. Indeed, the output voltage range of the synchronous rectifying converter is linked to the maximum acceptable `on` and `off` voltages provided by the secondary control winding. The `on` control voltage provided to the switch is proportional to the output voltage of the converter, i.e. to the voltage provided at the main secondary winding, whilst on the other hand, the `off` control voltage provided to the switch is negative and proportional to the input voltage of the converter, i.e. to the voltage at the primary winding of the transformer. Hence, the input and output voltages of the converter have direct impacts on the control voltage range provided by the control secondary winding which becomes generally incompatible for operating the electronic switch.